Current seismic prospecting methods include the use of acquisition devices distributed over a zone to be explored. They are intended each for collecting the signals received by at least one receiver such as geophones or hydrophones coupled to the ground, in response lo seismic signals emitted by a seismic source and sent back through subsoil discontinues. The signals are amplified, digitized and stored in local memories prior to being transmitted in real or delay in time to a central station through a transmission channel such as a cable, an optical fiber, a Rf channel, etc.
Seismic acquisition and transmission systems are for example described in patents FR-A-2,627,652 corresponding to U.S. Pat No. 4,908,803, FR-A-2,599,533 corresponding to U.S. Pat. No. 4,815,044, FR-A-2,538,561 corresponding to U.S. Pat. No. 4,979,152, or in patent application Ser. No. FR 92/12388 corresponding to U.S. Patent application Ser. No. 08/135,399 filed by the Applicant.
Data acquisition systems may include an automatic gain control suited to the type of digitizing apparatus used, allowing to be taken of the advantage of their conversion dynamic range, which is often high. Digitizing the signals in conventional acquisition chains is generally performed by taking successive samples from the analog signals by means of sample-and-hold circuits, and by applying them to an analog-to-digital converter. The gain may be adjusted by proceeding for example as described in patent FR-A-2,441,956 filed by the assignee. A steady gain is first being applied to a sample, followed by determining according to the corresponding digital word an optimum gain which has to be subsequently applied thereto to gain a full-resolution digital word.
In the most recent acquisition systems, the seismic signals are digitized by means of the oversampling technique. A sigma-delta type modulator oversamples the analog signals applied thereto and produces, at a high frequency, low-resolution digital words. A filter called a decimation filter is used for performing a sliding stacking of a certain number n of digital words from the modulator and for producing digital words of a higher resolution and at a lower frequency. A digitizing set of ibis type is for example described in patent FR-A-2,666,946 filed by the assignee.
The use of these delta-sigma modulators oversampling the signals continuously does not allow preamplifier gain controls to be achieved at cach held sample as was the case with the prior digitizing apparatus.
However, it is not possible in practice to impose a common gain value on the acquisition preamplifiers from a central station, notably because of the changes in to the emission-reception system during a single prospecting session. For some configurations, the seismic receivers which are the closest to the seismic source have an optimum gain and all of the conversion dynamic range of the converter (the signals they produce are coded with 24 bits for example). The seismic signals picked up by receivers which are further away from the source are weaker and are coded with a much smaller number of bits (10 to 13 for example). The seismic source is progressively shifted along the profile during a single recording session and previous distances between the source and the various receivers change which consequently modifies the conversion dynamic range effectively applied to a single "trace" during the successive cycles.